Seven years after his father attacked his mother, Henry Johnson is an aspiring singer-songwriter and still trying to come to terms with the attack. Photograph: Martin Godwin for the Guardian

To their friends, Nick and Nicky Johnson were "the Sunday supplement family", with seemingly idyllic English middle-class lives that could have been lifted off the pages of a glossy magazine. They had an apparently happy marriage and three beautiful children. They ran a successful language school and took luxury holidays in Tuscany or the south of France each year. They owned a rambling Victorian house in Ramsgate, and held late-night dinner parties, where the wine flowed up from their well-stocked cellar. So when their lives exploded in violence on 27 April 2004, no one could quite believe it.

There had been no hint of what was to come – either on that bright spring day, or in their 18 years of marriage – when Nick told Nicky he had a surprise for her. Using a tie as a blindfold, he ushered her into their garden, an acre of private square overlooked by elegant villas near the centre of town. "What it's going to be?" Nicky thought. Perhaps Nick had replaced the antique urns that had recently been stolen. The tie wasn't covering her eyes properly. She laughed: "I'm cheating because I can see."

The next thing she knew, she was slipping in and out of consciousness. She saw her husband sitting on the steps, his head in his hands. He had just hit her over the head with the blunt end of an axe, fracturing her skull in three places.

"I've got to stay alive so that the children are OK," she recalls thinking.

At that moment Henry, the couple's 16-year-old son, looked out of his bedroom window. He had come home to find the front door open, gone upstairs and heard a commotion outside. He rushed down, to see his mother lying on the gravel, her face barely recognisable. "Who did this?" he asked her again and again. Then he saw his dad, and the blank look in his eyes gave the answer. For an instant, Henry felt like getting a knife and putting it through him. Instead he called an ambulance.

"It felt like the worst thing that ever happened," he says. Now 24, and a talented, self-deprecating singer-songwriter, Henry is the subject of a forthcoming Channel 4 documentary in which he tries to understand the terrible, previously hidden impulse in his father that almost led to his mother's death, and confronts him about what happened. "It scares me that if a man can do something like that – one who I know is gentle, and has been a good man throughout my whole life – am I capable of that?" he says. "Is everyone capable of that? I could comprehend it if it was someone else. But I can't comprehend it. It's incomprehensible."

Nicky still feels that her husband "was only ever a wonderful father". "The most important people in his life were me and the children," she says. "He didn't have any aggression or meanness of any kind. He wouldn't even have smacked the dog. He was truly the last person on earth you would ever have expected to have lost it, and the last person anyone could imagine having wanted to harm me. I truly believe that nobody could have been such a perfect husband and perfect father and done something like that unless they were momentarily mad."

But if the attack appeared inexplicable, its aftermath was extraordinary too. As Nicky recovered in hospital, Nick was on remand, charged with attempted murder. She resolved to do all she could get him off: her overwhelming concern being to keep the family together. She encouraged Henry to visit him in prison.

"I didn't really want anything to do with him ever again, and when I visited him it was surreal," he recalls. "It was almost as if I didn't recognise him. We didn't say anything. Just cried. What is there to say? I hugged him but felt I shouldn't. It was a mixture of emotions … I came home and got off my face."

Psychiatric reports suggested that Nick had undergone a temporary psychotic episode triggered by the recent death of his father, whom he held responsible for his sister's suicide many years before. In court, Nicky stood up and gave him a glowing character reference, explaining how bad she thought it would be for the family if he was given a lengthy jail term. The attempted murder charge was dropped and Nick was eventually convicted of grievous bodily harm. After five months in jail, he came home.

While Henry's younger brother Felix, who had been largely protected from events, was happy to have his dad back, Henry wasn't. "It was as if nothing had changed. Like something had fallen off a shelf and it could just be put back up, even though it was broken." Looking back, however, he respects his dad for returning. "He didn't do a runner. He was trying to make everything all right." But something irrevocable had happened: the "safe bubble" and haven of Henry's youth had gone for ever, and his vision of his dad as "strong, caring and protective" had been shattered.

Nicky and Nick Johnson on holiday together. In 2004, Nick hit her over the head with an axe and fractured her skull in three places. Photograph: Channel 4

Before the attack, their relationship was great. "It was a real friendship. We would have debates about how the world works – in-depth talks about serious issues." He remembers them watching a documentary together and hitting pause on the DVD throughout so that they could discuss the fine points of the programme.

After Nick came back, Henry rebelled. "I started to go off the rails, but not in big way. Things like drawing all over my bedroom walls, smoking in the house, having friends round every weekend and making so much noise that Dad would come up and, in a gentle way, ask for the music to be turned down. I'd just ignore him. I thought: 'Fuck you, I'll behave how I want.' My whole philosophy was distorted. I forgot who I was, and trying to retrace the person I was before was too painful, so I just shut those boxes. I spent the next few years in that state. I lost a lot of friends and built up a wall around myself."

Music became his survival tool. Nicky is a classical guitarist, and as a boy, Henry had won a scholarship with Canterbury Cathedral choir and toured internationally. Now, skateboard in one hand, acoustic guitar in the other, he travelled and busked, pouring out his experiences into his own songs. "Music was cathartic, something I reverted to and expressed what I was going through, hopefully not in a self-indulgent, 'get the violin strings out' kind of way, but [something] more ambiguous. I wanted people to relate to me because I couldn't get it through speaking."

But at home, the fissures had not healed, and Nick still offered no real explanation for what he had done. "If I mentioned the incident, he would almost roll his eyes as if to say: 'You're not going to bring that up again,'" says Nicky, who had struggled to cope with the situation. "Dad never really helped me understand it," says Henry, "but I don't think he ever fully understood it himself."

After six months, Nick moved out and Nicky found a new partner. Henry thinks his mum had simply tried to do her best "under a huge amount of psychological strain and confusion". He says of her: "She's capable of huge understanding, but she has moments when she hates him. She's dealt with it by trying to make everything all right for everyone else, which is an honourable thing."

He still grapples with whether his dad's attack was pre-meditated, or a momentary mental aberration. "He's an intelligent man. His job involves negotiating and strategising, so if he'd wanted to kill her, he had so many other opportunities, like when they walked the dogs by the cliffs. Why do it there in broad daylight?"

Seven years on, a kind of resolution is nearer. "The incredible thing is that he can come round and we can all sit together at the dinner table and it's OK," says Henry. "It's almost like a death. You have to accept it and move forward even if there aren't answers to all the questions."